1. Field of the Invention
The present invention relates to drawing machines and more particularly to an X-Y plotter.
2. Description of Related Art
An X-Y plotter generally comprises first drive means for displacing a recording medium or sheet in a first direction (X); a write head carrying a writing tool; second drive means for displacing the writing tool in a second direction (Y) along a write path; a platen for supporting the sheet and having a horizontal central portion including the write path, and a front portion and a rear portion for supporting the sheet in front of and behind the central portion; and means for pressing the sheet against the platen, at least in the central portion thereof.
In a plotter of this type, a drawing is made on the sheet by combining displacement of the sheet in the first direction (X) and displacement of the writing tool in the second direction (Y) which is generally perpendicular to the first.
While a drawing is being made, each interruption of a plotted line is achieved by raising the writing tool from a lowered position in which it is in contact with the sheet to a raised position in which it is no longer in contact with the sheet.
A line is started by performing the opposite movement.
When the tool is in the raised position, it is essential to avoid any accidental contact between the sheet and the writing tip since that will mark the sheet and make the drawing unusable.
However, when the sheet is driven, for example by means of a rotary drum with the margins of the sheet being pressed against the drum by presser wheels, and no means are provided for holding the sheet against the support platen, the sheet deforms and it inevitably moves away from the platen under certain conditions, in particular when sudden accelerations are imparted to the sheet. By lifting in this way, the sheet may come into contact with the writing tool, even when the writing path is close to the location where the sheet is pressed against the drive drum.
The sheet may be held more reliably against the platen, or at least against the central portion thereof, by setting up suction beneath the sheet via openings formed through the platen. One such disposition is shown in copending U.S. patent application Ser. No. 055,699, filed May 29, 1987 (corresponding to French patent application Nos. 2,599,300 and 2,599,301 in the name of Benson). These documents also show front and rear portions of the platen in the form of skirts having sheet support surfaces each comprising a plurality of faces which are interconnected to one another by rounded portions, with a first face extending the central portion and with other faces extending the first face downwardly at increasing slopes relative to the horizontal.
It can happen, under certain circumstances, that holding the sheet down by suction in the central portion of the platen is insufficient. This happens when the sheet is caused to resonate under the action of its drive means, in particular when the drive means impose rapid displacement of the sheet back and forth along the X direction. The sheet is then subjected to high amplitude wave motion at its front and rear ends, and this motion propagates into the portion of the sheet supported by the platen and can be strong enough to lift the sheet off the platen along the write path by virtue of the low resistance of the sheet to deformation.
In order to prevent, or at least limit, the extent to which the sheet lifts off the platen, and thus avoid the drawbacks which result therefrom, proposals have been made to bend the sheet so as to increase its stiffness and thus increase its resistance to deformation. The sheet is bent by using a concave support platen with a V-shaped cross-section, or the like, combined with suction means for forcing the sheet to follow the shape of the platen. One such disposition is described in U.S. Pat. No. 4,369,453 and in an article by R. J. Kaplan and R. S. Townsend published in the "Hewlett-Packard Journal", November 1981, pp. 33-36.
Nonetheless, this prior disposition suffers from drawbacks.
The write path is situated substantially in the middle of the platen, i.e. in its hollow. Thus, in spite of the suction exerted through the platen, the sheet tends to lift off the platen when a drive force is applied to the sheet. The height of the sheet above the platen increases until it reaches a maximum and then decreases, while the sheet reaches its maximum speed at the end of acceleration in the X direction. If the writing tool is not raised high enough, or if the sheet is put into motion after too short a delay has elapsed relative to the tool being raised, the surface of the sheet may strike the tip of the tool. In order to prevent contact between the sheet and the raised tool, various different means are used.
A first means consists in raising the tool to a sufficient height. For a given degree of suction, the maximum height reached by the sheet depends on the nature of the sheet, on its thickness, and above all on its format (sheet size). Each format (sheet size) therefore corresponds to a different minimum height above which the tool must be raised. If it is desired to optimize the time taken to make a drawing regardless of format, it is necessary to provide means for automatically determining the minimum height to which the tool must be raised as a function of the format of the sheet, in order to increase the time taken for raising and lowering the tool only when that is necessary, i.e. when using large formats.
Other means consist in using mechanical means for preventing the sheet from striking the raised tool, for example a bumper fixed adjacent to the tip of the tool or an appendix which extends over the entire length of the plotter in front of and behind the platen and which accompanies the sheet as it moves in order to limit interfering movements of the sheet.
A second difficulty related to using a concave support platen lies in the need to find a compromise between the radius of curvature imparted to the sheet and the V-shape of the platen. The radius of curvature must be fairly small in order to ensure that the increase in stiffness provided by bending the sheet is sufficient for stabilizing sheet movement. However, the radius of curvature must be fairly large in order to avoid permanently deforming the sheet. Unfortunately, of the various sheet materials that may be used, some are liable to be permanently deformed if they are subjected to a relatively small radius of curvature, in particular paper having a density of 90 grams per square meter (g/m2).
Finally, another drawback of a concave support platen stems from its requirement of a segmented drum for driving the paper. If a continuous drum were to be used, i.e. a drum of constant diameter whose top generator line is flush with the surface of the platen and passes through an elongate opening formed therein, then the sheet could penetrate into the gap between the drum and the rear write path (i.e. the portion of the platen situated on the opposite side of the drum to the side having the normal write path) while the sheet is being loaded onto the plotter. In order to ensure that paper loading takes place properly, it becomes necessary to segment the drum, i.e. to provide portions of smaller diameter making it possible to maintain platen continuity level with said drum portions, thereby guiding the sheet while it is being loaded. Thus, the use of a concave platen prevents a continuous drum being used in a sheet-fed plotter because of the impossibility of ensuring that proper sheet feeding can be performed automatically.